At present, optical fiber transmission has been widely used in telecommunication system. In access layer, the commonly used modes are “optical fiber+category 5 cabling LAN (Local Area Network)”, “optical fiber+xDSL (x Digital Subscriber Line)” or “optical fiber+Coaxial-Cable”, in which all User Equipments (UEs) connect with optical fiber through cables rather than directly. Therefore, these modes can never completely shake off the limitations caused by transmission bandwidth of cables and make full use of the advantages of the optical fiber in bandwidth. Along with the parallel developments of voice, data and video services, bandwidth limitations of transmission cable have become a big obstacle to the development of telecommunications.
The “optical fiber+category 5 cabling LAN” mode provides subscribers with 10M/100M Ethernet network interface to support broadband data access, where the transmission distance of category 5 cabling is no longer than 200 meters.
The “optical fiber+xDSL” mode provides, via a copper twisted-pair, subscribers with interface of which the rate is from several hundred kbps to less than one hundred Mpbs, where the transmission distance is often less than 10 km and the transmission rates and transmission distances of different types of DSL (Digital Subscriber Line) are diversified.
The “optical fiber+Coaxial-Cable” mode provides subscribers with broadband services, which are mainly cable television (CATV) signals, as well as voice and data transmission in some countries through coaxial cable, where the rate is up to 40 Mbps but the transmission distance is also less than 10 km.
Therefore, in all the present optical fiber transmission modes, optical fiber is used for data transmission only in part of a communication network while other low bandwidth transmission media still exist between UE and optical fiber. As a result, on one hand, compared with the transmission bandwidth of optical fiber, the bandwidth of less than 155M of these low bandwidth transmission media such as copper cables can guarantee neither enough bandwidth that is needed for data transmission in the entire communication networks nor upgrading and capacity expansion of the communication network; on the other hand, the transmission distance of less than 10 km of these low bandwidth transmission media can also gravely limit the development of communication networks.
In order to overcome the shortcomings in the above commonly used optical fiber transmission modes, the FTTH/D (Fiber to the Home/Desktop) concept is put forward. At present, the major technique that is used in FTTH/D is BPON (Broadband Passive Optical Network). In terms of transfer protocol and transmission rate BPON can be further divided into APON (ATM Passive Optical Network), EPON (Ethernet Passive Optical Network) and GPON (Gigabit-capable Passive Optical Network).
The APON is one kind of optical fiber transmission technology that is based on ATM (Asynchronous Transfer Mode); APON system comprises OLT (Optical Line Terminal) at the service side, ONU (Optical Network Unit) at the subscriber side and ODU (Optical Distribution Unit) between said ONU and OLT, where ODU provides one or more optical transfer paths between the OLT and the ONU.
The EPON technology implements optical fiber transmission based on Ethernet and mainly comprises OLT, ONU and POS (Passive optical Splitter/Coupler); wherein, said OLT locates at CO (Central Office) and may provide interfaces between EPON system and the kernel data, video and telephone network of the service providers; the ONU locates at Customer Premises Equipment (CPE), and provides interfaces between the EPON and the data, video and telephone network of the subscriber; while said POS is a passive device that links the OLT and the ONU with the functions of distributing downlink data and converging uplink data.
Therefore, the present optical fiber transmission technology has provided relatively mature methods of data transmission through optical fiber and reliable technical supports for widespread applications of optical fiber transmission in network communication systems. However, the problems that need to be solved in applications of optical fiber transmission in an entire network communication system include not only the problem of how to transfer data in the optical fiber, but also that of how to packet the data to be transferred in a user terminal, how to distribute the data when the user terminal receives a data packet that contains different types of data, how to switch the data after the data have been transferred through the devices at the network side and etc.
In an existing communication network, optical fiber transmission technology is used for only part of the network. If optical fiber transmission is to be used in the entire communication network, adaptive adjustment is needed for the packet processing at the user terminal, for data switching after the data are transferred to CO, and for other corresponding functions in the existing network.
In an existing communication network, the switching portion of the network corresponding to optical transmission unit can only implement switching of bearer network level granularity, for instance, switching of VC4 or VC12, rather than the switching of service network level granularity, which means that the switch unit in the existing optical fiber transmission process can only accomplish part of the switching task in communications. The service network level granularity is the minimum bandwidth unit that is transferred and switched in the service layer of an existing communication network, and cannot be recognized, multiplexed or de-multiplexed by the bearer network, for instance, a transmission network with the granularity of 2M cannot recognize, multiplex or de-multiplex the 64 Kbps cross granularity in a voice service network. Moreover, the terminals in existing optical transmission networks are usually access devices or switching equipments rather than real client terminal devices.
An existing communication network can be divided into two layers, the bearer network and the service network, wherein the bearer network can be further divided into circuit switched SDH/WDM (Synchronous Digital Hierarchy or Wave Division multiplexing) transmission network and packet/cell switched IP/ATM (Internet Protocol/Asynchronous Transfer Mode) data network. An SDH/WDM transmission network can provide carrier-class services with excellent QoS (Quality of Service) and security characteristics while the complicated layered network architecture of its switching portion increases the cost of network construction and maintenance.
The above transmission network may be divided into access transmission layer, convergence transmission layer and core transmission layer, as shown in FIG. 1. The access transmission layer implements the access of dispersed subscribers to the local switch while convergence transmission layer and core transmission layer accomplish transmission across switches and transmission of long distance services. Therefore there is switching operation in each transmission layer.
The core switching layer of the above-said IP data network comprises high-rate routers and takes the form of Mesh network mostly while the convergence layer and access layer thereof comprise edge routers, three-layered switches, broadband access servers, and two-layered switches. So the network architecture is very complex.
The service networks in an existing communication network can be divided into voice, data and video service networks, which are above the bearer network and implement the processing and switching of the service layer, and the devices thereof include voice switches, two and three-layered data switches (L2/3), routers, and video switching platforms. Existing service networks usually construct and maintain their own networks respectively. In one word, layered network architecture and separate networking of the three major services make the overall networking cost and overall maintenance cost of existing communication networks remain high while problems in QoS of IP networks lowers the transmission quality of real-time services including video phone, video conference, and etc.
Therefore, applications of optical fiber transmission technology at present have the following problems:
1, the complicated layered network architecture makes the overall cost of network construction and maintenance remain high;
2, the separate construction of the layer of voice, data and video service networks in network communications brings about higher overall cost of network construction and maintenance, resulting in wastes caused by redundant network construction;
3, the relatively low bandwidth transmission media existing in the entire communication network, compared with the transmission bandwidth of optical fiber, can not meet the bandwidth requirement of subscribers, making it impossible to guarantee the QoS in network communications, for instance, there are relatively serious time delay and jittering in videophone and videoconference services;
4, the commonly used ring structure in network communications has relative poor protecting effects on data transmission;
5, neither application for bandwidth resources based on each call nor allocation of bandwidth resources on demand is supported in existing network communications, making it impossible to satisfy the diversified needs of users;
6, the electric magnetic radiation generated from copper transmission cables commonly used in communication networks damages the surrounding environment while surrounding electric magnetic radiation can also interfere with the signals transferred through copper cables.